The present invention relates to a choke assembly, in particular to a choke assembly having a plug and cage type arrangement, and to the use of the choke assembly in the processing of fluid streams. The present invention further relates to a wellhead assembly comprising the choke assembly, in particular a subsea wellhead assembly. The present invention further provides a method of operating a choke assembly and for controlling the pressure of a fluid stream.
Choke assemblies are well known and widely used in an extensive range of fluid processing applications. One particular application of chokes assemblies is in the processing of fluid streams produced from subterranean oil and gas wells, in particular in installations associated with the wellhead. The principle function of a choke assembly is to control the pressure and flowrate of the fluid passing through the assembly. This is achieved, in general, by providing the choke assembly with a fluid flow passage having a variable resistance to flow or pressure drop for fluid flowing along the fluid flow passage. By changing this resistance to flow or pressure drop, the flowrate and pressure of the fluid stream may be controlled, for example to regulate a fluctuating or changing fluid pressure or to tailor the pressure of the fluid stream to meet downstream processing requirements.
One common choke assembly comprises a so-called ‘plug and cage’ arrangement. This assembly has a cage, typically cylindrical in form, comprising a plurality of holes or apertures therethrough for the passage of fluid. A plug, again generally cylindrical in form, is provided so as to be moveable with respect to the cage, the plug being disposed to be moveable to cover or close the apertures in the cage. The plug may be moved with respect to the cage between a closed position, in which all the apertures in the cage are covered, thus preventing fluid from flowing through the choke assembly, and a fully open position, in which all the apertures in the cage are open and available for fluid flow. Moving the plug with respect to the cage from the closed to the fully open position progressively uncovers the apertures in the cage, thus increasing the cross-sectional area available for fluid flow. In this way, the flow rate and pressure of the fluid may be varied and controlled. In the closed position, the end sealing portion of the plug contacts a seat formed in the choke assembly, so as to provide a fluid-tight seal, preventing the passage of fluid past the plug and cage. The plug may be arranged coaxially within the cage or coaxially exterior of the cage, known in the art as an external sleeve.
US 2007/0095411 discloses a fast closing well choke. The choke, described as being conventional, has a cage outer sleeve and a cage inner sleeve, both comprising cylindrical members arranged coaxially. Apertures extend radially through the inner and outer sleeves. A cylindrical plug is moveable within the inner sleeve. The figures of US 2007/0095411 show the choke in cross-section only and two rows of apertures are visible in the cross-sectional views. However, no details are shown or described of the distribution of the apertures around the cage sleeves.
Conventional chokes generally have a plurality of apertures through the cage, the apertures being of a variety of diameters and arranged in a staggered pattern around and along the cage. Further examples of conventional chokes are the control chokes commercially available from Cameron Flow Control, Cameron International Corporation, Texas, United States of America.
It will be appreciated that the efficiency of the choke assembly in controlling the fluid flowrate and pressure is dependent upon the integrity of the seal between the plug and the cage and the seal between the end of the plug and the seat in the closed position. A particular problem exists with the aforementioned conventional design of plug and cage choke assembly, in that the flow of fluids through the apertures past the end of the plug erodes the surfaces of both the plug and the cage. In particular, the fluid stream acts to erode and cut into the sealing end portion of the plug. Erosion of the sealing end portion of the plug reduces the effectiveness of the plug in sealing the apertures in the cage, in turn reducing the efficacy of the choke in controlling fluid flow and pressure. In addition, the erosion of the sealing end portion of the plug reduces the efficiency of the seal between the end portion and the seat of the plug when in the closed position. This in turn results in the choke assembly allowing fluid to continue flowing when in the closed position. In a case of long term and persistent erosion, the choke assembly can be rendered entirely inoperative in preventing or controlling the flow rate and pressure of the fluid stream.
It will be further appreciated that the rate and extent of erosion will depend upon the flowrate and composition of the fluid stream being handled. Fluid streams containing significant amounts of entrained solids will give rise to a very high rate of erosion of the both the plug and the cage, very rapidly reducing the efficiency of operation of the choke assembly.
A choke assembly in which significant erosion has taken place will require servicing and/or replacement. This is preferably avoided in any fluid processing installation. However, the servicing of choke assemblies is a particular problem in the case of processing installations in remote or difficult-to-reach locations. This problem is particularly acute in the case of choke assemblies installed in subsea wellhead installations.
The production of fluid streams from a subterranean well is controlled by at least one, generally a plurality, of choke assemblies mounted in the wellhead installation. The replacement of a choke assembly in a wellhead installation may be particularly difficult if the choke assembly is required in order to maintain the integrity of the well. This problem is exacerbated in the case of wellhead installations installed on subsea wells, where the wellhead installation is located on the seabed. The great depths at which the wellhead installations are deployed make access for servicing and maintenance very difficult. This is a particular problem with subsea choke assemblies, as the fluid streams produced from subterranean wells generally comprise substantially amounts of entrained formation solids, such as sand and gravel, together with debris that may be generated as a result of drilling and other downhole operations.
It will therefore be appreciated that there is a need for choke assemblies, in particular those to be installed in subsea locations, to have a high resistance to fluid erosion.
One approach taken in the art to reduce the erosion of plug and cage assemblies in chokes is to form the components exposed to the fluid stream from erosion resistant materials, for example tungsten. Examples of chokes incorporating such technology are known in the art and are commercially available, for example ex. Cameron Willis. In addition to erosion resistant materials, the chokes employ a multi-stage trim design, in which a plurality of coaxially arranged cages or sleeves are used, the sleeves having holes that are specifically aligned so as to force individual fluid jets to change direction repeatedly. In this way, it is alleged that the high energy, velocity and turbulence of the fluid stream are dissipated and controlled within the confines of the trim, avoiding erosion damage to the pressure containing boundaries of the choke assembly.
For applications involving fluids with significant entrained solids, it is common practice in the art to form various components of the choke assembly from materials having a high resistance to erosion, in particular those components on which the fluid stream is likely to impinge during operation. Thus, the aforementioned commercially available chokes have critical components formed from tungsten carbide. This material is expensive and particularly difficult to machine and form into complicated components, such as may be required for a choke assembly.
Tungsten, often employed to provide erosion resistance to choke components, is very brittle. As a result, tungsten components, such as the cage and the like, can crack if impacted by a large solid particle, such as may be produced by a subterranean well during a variety of operations. If there is a likelihood of such damage arising, the tungsten cage may be surrounded by a softer material, such as stainless steel, to prevent particles impacting directly onto the tungsten components.
In addition, the chokes comprise a sacrificial plug nose disposed on the end of the plug. The plug nose is impacted by the fluid stream during normal operation, instead of other critical components, such as the sealing surfaces of the plug. In use, the sacrificial plug nose is eroded by the aggressive fluid stream, in place of the more sensitive sealing components. While the provision of such a feature may prolong the lifetime of the choke, erosion will nevertheless occur, leading to the eventual failure of the choke, requiring it to be replaced and/or serviced.
Finally, the chokes are provided with a so-called ‘dead band’, allowing the stem of the choke assembly to travel between 5 and 10% before any significant flow can pass through the choke. This arrangement is intended to move the seating face of the plug, that is the end surface of the plug which in use defines a boundary of the flow passage through the choke and seals the plug against the seat, away from the main flow path, such that the fluid is directed onto the specially designed sacrificial areas of the choke assembly, such as the nose. Again, this arrangement still requires critical components to be directly in the path of fluid flow through the choke, leading in turn to erosion of the choke components and eventual failure.
Accordingly, there is a need for an improved choke design that reduces the erosive effect of fluid streams on the critical components of the choke assembly, in particular the plug and the cage, whether the plug is within or exterior to the cage.